The present invention relates generally to tube plugs and plug installation techniques and more particularly to thimble type plugs and the use of high pressure expansion mandrel plug installation tooling and technique to stop leaks in tubes found in boilers, condensers and other shell-and-tube type heat exchangers.
Some types of tube plugs used are tapered fiber (wooden or composite material) type plugs, one piece tapered metal plugs, two-piece metal tapered pin and collar plugs, multi-piece breakaway plugs, explosive plugs and thimble plugs. The tapered fiber plugs are usually limited to low pressure and temporary applications and in many cases are not compatible with the medium that would come in contact with the plug. Tapered metal one piece and two-piece (pin and collar) type plugs rely on mechanical methods for setting the plugs in the tubes with little or no control on the radial pressure applied to the tube. They are typically installed using a hammer type device. There is no way to reliably control and measure the radial force that these type plugs exert on the ID of the tube and the surrounding tubesheet ligaments during installation. This lack of radial pressure control can result in a tubesheet ligament being over stressed (plastically deformed) or not enough pressure applied to stop the leak. These forces can vary greatly from plug to plug even when installed by the same person using the same installation tools and techniques. One plug can be too loose and leak or even fall out, while the next plug can be too tight and cause permanent ligament damage which can generate leaks in the surrounding tube-to-tubesheet joints. Other potential problems with tapered metal plugs are described in Stanley Yokell's book “A Working Guide to Shell-and-Tube Heat Exchangers”. It states Quote: “they create a crevice behind the contact surface between the plug and tube, which can lead to crevice corrosion; and the tensile stresses they generate in the tubesheet may sensitize it to stress corrosion cracking.” A disadvantage of the two-piece tapered plug is that it requires a tight friction fit in three areas to work properly. One is between the tapered pin (inner piece) and the ring (outer piece), second is between the OD of the ring and the ID of the tube and last is the surface between the tube OD and the tubesheet hole. Another disadvantage is the outer ring needs to have an initial clearance gap between it and the tube ID for the plug to be inserted. This requires a substantial deformation of the ring to engage the tube ID with sufficient force to create the interfacial pressure needed to hold it in place and seal the leak. It is possible that because the shoulder of the ring rests against the tube end during installation that the driving force necessary to seat the plug and expand it may cause the tube itself to be dislodged from its seat and create another leak path. Multi-piece breakaway plugs have a very small effective expansion range and therefore must be precisely sized to work properly: A common problem is when an undersized plug is selected for use. During installation the breakaway plugs become rapidly work-hardened during the expansion process. The resistance and resulting friction forces will cause the breakaway of the plug parts to take place before the necessary radial forces are reached that should hold the plug in place and seal the leak. The results are plugs that may loosen and fail during the operation of the unit.
Explosive friction fit plugs must be installed by explosive certified personnel and cannot be used in many applications because of the hazardous environments that the vessels are located in such as chemical plants and refineries. Even if they can be used, it may require whole areas of the plant to be shut down and personnel removed from the area for safety reasons during their use. Another down side of explosive plugs is the extensive cleanup required to remove the explosive residue from the vessel to prevent plant systems contamination prior to bringing the vessel back online.
Thimble type plugs are typically a one piece plug that resembles a thin tube with a closed end or “blind hole.” When installed using mechanical rollers the plug must fit snuggly in the tube hole to begin with to prevent it from spinning during the initial rolling process. In all of the applications discussed with the exception of explosive plugs it has been proven that when the ID of the tube hole being plugged is even slightly oval shaped it is difficult to get the plug OD to conform to the tube ID and provide a reliable sealing surface.
In the patent to Russell D. Wasson and David A Vossbrinck, U.S. Pat. No. 5,901,594, issued on May 11, 1999, for High Pressure Expansion Mandrel with Cams Engaging Oppositely Directed Ends of an Expandable Segmented Ring, there is disclosed a high pressure mandrel for joining a metal tube to a wall of a metal sheet surrounding an annular bore of the metal sheet. Spaced O-rings are disposed on a shaft of a mandrel that is disposed axially in the metal tube. Outboard of the O-rings, backup rings are respectively disposed on the shaft. Outboard of the backup rings are disposed segmented ring assemblies which are also disposed on the shaft. Facing opposite ends of each segmented ring assemblies are cam rings. The cam rings are disposed on the shaft. High pressure fluid by means of a conduit in the mandrel shaft provides pressure to the inner wall of the metal tube to expand the tube radially outward. Simultaneously, the high pressure fluid urges the O-rings outward toward the associated backup rings and in turn the backup rings are urged outwardly toward the associated segmented ring assemblies, respectively. Facing each of the segmented ring assemblies are cam rings. Each set of cam rings expand radially its associated segmented ring assembly in response to the high pressure fluid.